1. Field of the Invention
The present invention relates to a photoconductive screen having such a construction that a pattern of electrostatic charges may be formed thereon to control a flow of charged particles, for example, positive ions passing thereacross so a certain electrostatic latent image may be formed in a chargeable layer, for example, made of a dielectric or photoconductive material under exposure to the above flow of charged particles.
2. Description of the Prior Art
For example, Japanese Patent Publication No. 138734/1978 discloses a photoconductive screen 7 having a biasing layer, as shown in FIG. 1, comprising a screen substrate 1, a photoconductive layer 2 on one side thereof and an insulating layer 3 and conductive biasing layer 4 on the other side thereof. Since in the above photoconductive screen the photoconductive layer 2 and insulating layer 3 are formed on different sides of the substrate 1, it is difficult to fabricate the screen in the form of an endless drum and further there is a high possibility that the biasing layer 4 formed is in contact to the substrate 1 by accident, making biasing impossible.
More specifically, to fabricate the photoconductive screen of FIG. 1, one side of the substrate 1 is coated using a spray gun 8 to form an insulating layer 3 as in FIG. 2A. Next, aluminium or other metal is deposited on the insulating layer 3 by vacuum evaporation to form a conductive biasing layer 4 and then the other side of the substrate 1 is coated using a spray gun 9 to form a photoconductive layer 2, as shown in FIG. 2B. Accordingly, if the substrate 1 of the form of a screen drum is used and individual layers are formed while revolving the substrate as in FIG. 3, it is difficult to have a sufficient space between the substrate 1 and spray gun 8 inside in formation of the insulating layer 3 and also between the substrate 1 and an evaporation source 10 inside as hatched in the figure in evaporation to form the biasing layer 4. For this reason, the insulating layer 3 has many irregularities in thickness with inferior quality of coating. Further, since the spray solution sticks and turns around to readily come on the other side of substrate, the resultant photoconductive screen, when used, exhibits poor control of the ionic flow. In addition, because of an insufficient space from the evaporation source, when the biasing layer 4 is formed by evaporation, vaporized metal sticks and turns around to readily come on the other side of substrate 1 and deposits there. As a result, there are often formed closed circuits between this layer and substrate 1. In this case, no biasing effect can be expected.
For the reasons as mentioned above, it is very difficult to fabricate a photoconductive screen of FIG. 1 of good quality with a substrate 1 that is provided in the form of drum as in FIG. 3. Therefore, it becomes unavoidable to start with a flat piece of screen substrate 1 and form individual layers on the two sides thereof by coating and evaporation. In this case, however, since the flat screen substrate 1 poorly maintains its shape (i.e., it is liable to deformation), it is necessary to hold its four sides with a frame 11 (FIG. 4) and therefore the areas covered with the frame become ineffective. Namely, if such flat screen substrate, after formation of individual layers, is curled to a cylindrical form to provide a photoconductive screen in the form of drum, it is necessary to cut away the framed portion at least at a pair of opposite edges of substrate 1 (namely, edges that are directed along the width of formed drum) to join a pair of opposite cut ends thereof.
Further, even after a screen is shaped in the form of drum, it is not possible to directly join the opposite ends of the substrate 1, so they must be joined by means of a bonding member 12, which provides a seam that performs nothing as the photoconductive screen. This seam thus necessarily limits the position of the leading edge of the effective imaging area and copy paper. As a result, every time the drum photoconductive screen 7 turns a turn, a continuous image is formed only in a range that corresponds to a single turn of drum and therefore the copy paper supplied is practically limited in size. Namely, if the photoconductive screen 7 is turned continuously more than a single turn for image reproduction in various sizes of copy paper, the above seam appears itself as a linear stripe or stripes in the copy image. For this reason, to reproduce an image of large size in a copy paper, the drum photoconductive screen 7 used must necessarily have a correspondingly large drum diameter, which needs a copier system of large size. Further, in each copy process, the stop position of the drum screen must always be fixed relative to the seam. This requires more complicated control of the drum driving system and the drum driving system itself must be given extra motions, which prevents highspeed copy reproduction.